Oligocarbonate polyols are important preliminary products used in the preparation of plastics, paints and adhesives. They can be reacted with isocyanates, epoxides, (cyclic) esters, acids and acid anhydrides. See, for example, DE-A 1955902 and EP-A 0 343 572. They can also be prepared from aliphatic polyols by reacting with phosgene (See, for example, DE-A 1 595 446 and U.S. Pat. No. 4,533,729), bis-chlorocarbonic acid esters (See, for example, DE-A 857 948), diaryl carbonates (See, for example, DE-A 1 915 908), cyclic carbonates (See, for example, DE-A 2 523 352, DE-A 1495299 and U.S. patent application Ser. No. 787,632) and dialkyl carbonates (See, for example, DE-A 2 555 805, EP-A 0 343 572 and EP 0 533 275).
It is known that, when reacting aryl carbonates (such as diphenyl carbonate) with aliphatic polyols (such as 1,6-hexanediol), it is possible to achieve an adequate reaction conversion only by removing the alcohol compound (e.g., phenol) to shift the equilibrium of the reaction (See, for example, EP-A 0 533 275).
If, however, alkyl carbonates (e.g., dimethyl carbonate) are used, then transesterification catalysts such as alkali and alkaline earth metals, their oxides, alkoxides, carbonates, borates and salts of organic acids (See, for example, U.S. Pat. Nos. 2,210,817 and 2,843,567, DE-A 2 523 352, DE-A 1 495 299, EP-A 0 49 303, EP-A 0 754 714, EP-A 0 533 275 and WO 97/03104) are typically used.
Additionally, tin and organotin compounds such as dibutyltin, dibutyltin laurate and dibutyltin oxide (See, for example, DE-A 2 523 352, EP-A 0 364 052, EP-A 0 600 417, EP-A 0 343 572 and EP-A 0 302 712), and compounds of titanium, such as titanium tetrabutylate, titanium tetraisopropylate and titanium dioxide, are preferably used as transesterification catalysts (See, for example, U.S. Pat. No. 2,843,567, EP-A 0 849 303, EP-A 0 343 572, EP-A 0 424 219 and EP-A 0 754 714).
However, known transesterification catalysts used for the preparation of aliphatic oligocarbonate polyols by the reaction of alkyl carbonates with aliphatic polyols have some disadvantages.
When strong bases such as alkali and alkaline earth metals and their alkoxides are used as transesterification catalysts, it is necessary, once oligomerization begins, to neutralize the products in an additional step (See, for example, EP-A 0 533 275). If, on the other hand, tin compounds are used as catalysts, undesirable discoloration (such as yellowing) can occur when the resulting product is stored. This is due, inter alia, to the presence of Ti(III) compounds together with Ti(IV) compounds which are simultaneously present or is caused by the complex-forming tendency of titanium.
Additionally, organotin compounds are recognized as potential carcinogens in humans. They are therefore undesirable constituents which remain in oligocarbonate polyol secondary products if compounds such as dibutyltin oxide and dibutyltin laurate are used as catalysts.
Furthermore, it is not possible to lower the reaction temperature, which is usually between 150° C. and 230° C. (See, for example, EP-A 0 533 275 and EP-A 0 364 052), in order to avoid the formation of by-products, such as the formation of ethers and vinyl groups, which can form at an elevated temperature. Such undesirable end groups act as chain terminators for subsequent polymerization reactions, such as the reaction of polyurethane with polyfunctional (poly)isocyanates, and lead to a lowering of the crosslink density and hence to poorer product properties, such as resistance to solvents and acids (See, for example, DE-A 1 915 908). On the other hand, a more rapid transesterification reaction results in an increase in the space/time yield (lowering of the vessel occupancy time) and hence an increase in processing efficiency, which could not be previously achieved.
An object of the present invention is, therefore, to provide suitable catalysts for the transesterification reaction of organic carbonates, especially dialkyl carbonates, with aliphatic polyols for the preparation of aliphatic oligocarbonate polyols, wherein the catalysts have a higher activity and do not exhibit the disadvantages mentioned above.
Surprisingly, the object of the present invention has been achieved with the use of organic and inorganic compounds of the rare earth metals and metals of group III B of the periodic table of the elements.